Engine systems may utilize recirculation of exhaust gas from an engine exhaust system to an engine intake system (intake passage), a process referred to as exhaust gas recirculation (EGR), to reduce regulated emissions and improve fuel economy. An EGR system, such as a low-pressure EGR system, may include various sensors to measure and/or control the EGR. As one example, an engine intake system may include an intake gas constituent sensor, such as an oxygen sensor, which may be employed during non-EGR conditions to determine the oxygen content of fresh intake air. During EGR conditions, the sensor may be used to infer EGR based on a change in oxygen concentration due to addition of EGR as a diluent. One example of such an intake oxygen sensor is shown by Matsubara et al. in U.S. Pat. No. 6,742,379. However, the accuracy of EGR estimates using the intake oxygen sensor may be reduced during boosted engine operation and purge conditions when hydrocarbons are flowing through the intake system. EGR flow may then be estimated using alternate EGR sensors. For example, the EGR system may also include differential pressure over valve (DP) sensor positioned around an EGR valve for estimating EGR flow based on a pressure difference across the EGR valve and a flow area of the EGR valve. EGR flow estimates may then be used to adjust a position of the EGR valve and therefore adjust an amount of EGR provided to the engine.
As one example, a flow area of the EGR valve may change due to soot accumulation, or other build-up, on the EGR valve. This change in EGR valve flow area may impact the EGR flow estimate, and thus EGR control, based on measurements from a DPOV system including the DP sensor. However, the inventors herein have recognized that when the EGR valve closes, an edge of the valve may cut into the valve seat and changes in valve lift due to soot build-up may not be detected. Thus, changes in the EGR valve flow area affecting EGR flow may not be compensated for with end-stop learning diagnostics using the DPOV position sensor, thereby resulting in inaccurate EGR flow estimates when using the DPOV system.
In one example, the issues described above may be addressed by a method for indicating soot accumulation on an exhaust gas recirculation (EGR) valve based on a difference in EGR flow estimated, during a first condition when the engine is not boosted, with an intake oxygen sensor and with a pressure sensor coupled across the EGR valve. In this way, changes in a flow area of the EGR valve due to soot accumulation may be detected and subsequent EGR flow estimates may be corrected based on the soot accumulation, thereby increasing an accuracy of EGR flow estimates and resulting engine control.
As one example, a first EGR flow estimate may be determined using an intake oxygen sensor when the engine is not boosted and purge flow is off. A second EGR flow estimate may be determined with a pressure sensor coupled across the EGR valve, such as a differential pressure over valve (DP) sensor. The difference between the first and second EGR flow estimates may then be used to determine a change in flow area of the EGR valve due to soot accumulation. More specifically, the second EGR flow estimate may be based on an output of the DP sensor and the flow area of the EGR valve, where the flow area of the EGR valve is estimated based on a known cross-section of the EGR valve and an EGR valve position based on an output of an EGR valve position sensor. The change in EGR valve flow area (due to soot build-up) may then be based on the difference in the first and second EGR flow estimates, an expected EGR valve flow area, and a first EGR flow estimated with the intake oxygen sensor. The expected EGR valve flow area may be based on an output of the EGR valve position sensor and an EGR valve lift correction, where the EGR valve lift correction is learned during an EGR valve end stop and thermal compensation learning routine. In one example, the EGR valve ends stop and thermal compensation learning routine may determine a change in flow are of the EGR valve due to a temperature difference between the stem and body of the EGR valve. In this way, changes in EGR valve flow area affecting EGR flow estimates using the DPOV method described above may be determined and used to correct EGR flow estimates. In this way, more accurate EGR flow estimates may be used for engine control.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.